PRIOR ART
Road and ground constructions consist essentially of a wearing course disposed on the top, and, below it, a base course formed in varying thicknesses from a well-defined sand or gravel material. In cases where the ground layers have a particularly low carrying or load-bearing capacity, subbases can be added, these also being formed from a defined composition. A characteristic feature of conventional road and ground surfacings is that the base courses can only tolerate small tensile stresses. The function of the base course is essentially one of load distribution or, in other words, increasing the surface area influenced by the point loads which are exerted on the wearing course to an acceptable level. The tensile stresses which are formed in the base course are dissipated as friction in the underlying earthen mass.
Conventional road surfacings are made up of base courses and wearing courses whose bulk density is at least as great as that of the underlying ground. Consequently, road surfacings having considerably different bulk densities exist for various soil types. For example, well-graded, packed, sandy gravel has a bulk density of 1800-2000 kg/m.sup.3 ; clay, 1500-1600 kg/m.sup.3 ; and peat, 1000-1100 kg/m.sup.3.
Bitumen stabilization is used to increase the tensile strength of base courses and, especially, the ability to withstand short-term loads. Various construction procedures including, for example, the use of fiber fabric mats, increase the tensile strength of both the base course and the underlying earthen mass. Cement or lime stabilization of the underlying ground, or the like, is primarily intended to increase rigidity. At the same time, the tensile strength also increases. Other measures for increasing the load-bearing capacity of the base courses and for transferring tensile stresses include laying out horizontal piles with end anchors or grillages of wood. Concrete, either plain or reinforced, is also used as a construction material. The concrete generally constitutes a wearing course, but also contributes to distributing point loads along the underlying ground layers. By virtue of the reinforcement, the tensile strength of the concrete is considerably improved. Even if the density of the concrete is 2300-2400 kg/m.sup.3, the reduced thickness required for the base course results in a corresponding reduction in the intrinsic load exerted by the entire construction. Plastic-molded concrete tends to shrink over time, thereby causing uncontrollable crack formations to occur. Therefore, concrete surfacings are generally provided with joints intended to function as crack inhibitors. With such joints, the capacity of the wearing course to tolerate tensile stresses caused by bending moment is reduced. In order to prevent extensive settling due to such joints, the base course is typically chosen to be relatively thick, thereby increasing the load exerted by the entire construction. The load can be reduced to some extent by producing the wearing course from light ballast concrete. In the United States, for example, light ballast concrete with a density as low as 1600 kg/m.sup.3 has been used with good results. Concretes having lower densities, however, have too little abrasion resistance and are quickly and easily worn down by traffic.
The load exerted on the underlying earthen masses can be reduced even further in several ways. Materials with low bulk density, such as slag, haydite and cellular plastic, have been used to reduce the weight of road embankments. Piles driven into the ground may also be used to transfer the load from the roadway down to deeper-lying earthen layers having a higher load-bearing capacity and rigidity than those lying above. The piles can be provided with pile helmets, or a reinforced, continuous concrete slab can be cast and supported by the piles. The base course and wearing courses are then formed above the slab. In this arrangement, the load-bearing capacity of the underlying earthen layers is not utilized, the construction being comparable to the absorption of all loads in the supports of a bridge. At the same time, loading and drainage of the ground results in a packing effect in which the porosity decreases, as does the pore water pressure. For ground having a low load-bearing capacity, the upper ground layers are often drained by a system of pipes, and a preliminary load is applied by the laying of subbase and base courses. Vertical drainage is also performed in order to shorten the consolidation time upon loading. In this way, any extensive settling of the ground occurs before the wearing course is applied. Such construction is typically provided with pipes for leading surface water away and for preventing a rise in ground water. In many cases where the ground is extremely inhomogeneous, the worst earthen masses are removed before pre-loading is applied. The ability to detect those areas which may result in particularly extensive settling is enhanced with pre-loading and simultaneous drainage.
Additional deformation in the form of so-called frost damage may also occur as the ground freezes. Such deformation may appear where frost protection material is located under road and ground constructions. The damage occurs when the ground water is conveyed in capillary fashion in fine-grained earth up to the freezing zone where an accumulation takes place and ice crystals are formed. Freezing occurs more easily when the surface construction is exposed, as with snow plowed roads having insulating snow banks along the sides thereof. The material in the roadway has little heat-insulating power so that the freezing is concentrated in the areas under the roadway itself. In order to prevent frost damage, the frost-susceptible material must be removed and the ground drained under the construction by pipe drainage. In order to improve the heat-insulating power of the roadway, insulating materials such as haydite and slag can be used.
The ability of a soil type to absorb loads with subsequent deformations depends on the particle size and distribution, the degree of compaction and the pore water pressure in the intermediate space between the soil particles. In loose soil types, such as clay and peat, the soil structure itself can only bear a load for which the soil layer has previously reached an equilibrium, i.e., the pre-consolidation pressure. When the load increases beyond this pressure, the excess load is initially absorbed by the pore water pressure in the soil layer. This pressure changes with time, the change depending on the permeability of the soil, otherwise known as the dewatering rate. As a certain volume of water is squeezed out of the soil, a corresponding deformation or settling in the ground layers occurs. This settling is irreversible.
The load-bearing capacity and rigidity of the earthen masses increase with increasing depth as overlying layers become compacted and dewatered over time. Thus, a so-called consolidation takes place. If a certain critical load is exceeded, the deformations increase quickly. When designing constructions on cohesive soils, either the load must be below this critical load, or the load must be transferred via piles down to soil layers having greater load-bearing capacity. Since ground layers are heterogeneous, the critical load tends to vary from place to place.
Since the surface of road and ground constructions lies above the adjacent ground to permit water runoff, loads exerted on these constructions are transferred to underlying ground layers. The pore water between the soil particles in the ground layers is drained off and remaining deformations appear. Settling in the ground layers is more pronounced with low pre-consolidation pressure. Such ground layers require, moreover, thicker base courses in order to increase the area influenced by the point loads exerted on the wear surfaces. This in turn results in greater overall loading on the ground layers and, hence, increased settling.